1. Field of the Invention
The present invention relates to an optical device such as monoculars and binoculars.
2. Description of the Related Art
In general, hermetically sealed waterproof optical devices, for example, an underwater camera disclosed in Japanese Patent Laid-Open No. 2002-122929, include a transparent housing or a transparent window. Needless to say, such waterproof optical devices must prevent water from entering inside. In view of their applications, for example, the use of such optical devices at low temperature causes fogging due to condensation of water vapors. In such a case, the main function serving as optical devices including the above transparent portion is impaired. Therefore, the water vapor pressure inside of optical devices must be controlled so as to not exceed a predetermined value at any given time.
Hitherto, in addition to the above underwater camera, for example, waterproof binoculars are known as hermetically sealed waterproof optical devices. For example, Japanese Patent Laid-Open No. 7-209589 (corresponding to U.S. Pat. Nos. 5,677,789 and 5,883,738) discloses such waterproof binoculars.
Such waterproof binoculars generally have exterior parts composed of a metal such as aluminum or a magnesium alloy to prevent the permeation of moisture (including water vapors). In addition, an inert gas such as nitrogen is filled in the waterproof binoculars in order to remove air containing water vapors due to the environment in the assembly process. In some recent waterproof binoculars having an image stabilizer, the exterior parts are composed of a resin.
Regarding underwater cameras, it generally suffices to achieve the waterproofing only when the underwater cameras are actually used in water. Subsequently, water vapors and water droplets can be removed when a back cover is opened in order to replace a roll of film. Therefore, the permeability of water vapors need not be considered. This is also true of optical devices such as digital cameras using an additional underwater housing, that is, a hermetically sealed casing having a transparent window for the purpose of use in water.
Regarding the above known hermetically sealed optical devices, in particular, since users barely perform any maintenance after using the devices, the water vapor content in the devices must be constantly low, and in addition, a reasonable moistureproof structure must be provided. According to the moistureproof structure of the known hermetically sealed optical devices, as described above, the exterior parts composed of a metal can provide an excellent gas barrier property and high strength. However, in terms of low cost and weight reduction, such a moistureproof structure is not necessarily satisfactory, and thus preventing the popularization to the public.
Even when the optical devices include exterior parts that are composed of a metal and provide the excellent gas barrier property, the following phenomenon prevents the improvement in moistureproof performance. The use of a water retention material inside the devices generates water vapor as time goes on. In such a case, as will be clearly described in calculation examples below, the presence of a large amount of water vapors, which significantly exceeds the saturated water vapor at a low temperature, must be accepted.
Furthermore, for example, waterproof binoculars with an image stabilizer including exterior parts composed of a resin instead of a metal have a low gas barrier property. Unfortunately, after such waterproof binoculars are used, the waterproof performance is deteriorated over a short term. In addition, a large apparatus is required for filling inert gas such as nitrogen. Therefore, a place where, for example, the maintenance can be performed is limited.